The present invention relates to a process for the production of cellulosic flat films in accordance with the generic part of claim 1.
Some suggestions have already been made regarding the production of cellulosic flat films according to the so-called xe2x80x9camine oxide processxe2x80x9d, in which the flat film is made from a solution of cellulose in an aqueous tertiary amine oxide. Since N-methyl morpholine-N-oxide (xe2x80x9cNMMOxe2x80x9d) is preferably used as the tertiary amine oxide, the term xe2x80x9cNMMOxe2x80x9d is used in the following to represent all the suitable tertiary amine oxides.
Thus EP-A 0 574 870 describes amongst other things the production of a cellulosic film by means of extruding a solution of cellulose in aqueous NMMO through a die which has an extrusion gap, the width of which lies in the range of 50 to 200 xcexcm. In the examples provided in EP-A 0 574 870 cellulose solutions are extruded through dies with a gap length of 4 cm and a gap width of 50 xcexcm, 100 xcexcm, 150 xcexcm, 200 xcexcm and 300 xcexcm.
EP-A 0 574 870 emphasises that the highest possible shear gradient should be reached in the moulding tool whereby high tenacity values are to be reached in the moulded body even when the drafting in the air gap is low. According to EP-A 0 574 870 this should be achieved by keeping the diameter of the extrusion tool (in the case of films the width of the extrusion gap) at a low level. In this connection, however, EP-A 0 574 870 only examines the tenacities of the films produced in the longitudinal direction.
From PCT-WO 97/37392 the production of a flat film according to the amine oxide process via a die with an extrusion gap with a length of 1 m and a width of 100 xcexcm is known.
The laid-open Austrian patent application A 705/97 describes the production of cellulosic flat films according to the amine oxide process by means of the extrusion of the cellulose solution via a die, the extrusion gap of which has a length of at least 40 cm. In the examples provided in A 705/97 the width of the extrusion gap equals respectively 100 xcexcm, 300 xcexcm, 500 xcexcm or 1000 xcexcm.
In many fields of application for flat films high tenacities transverse to the extrusion direction are desired in addition to good tenacities in the longitudinal direction. With regard to the production of flat films with larger widths in particular, e.g. as of 30 cm and more, it is, however, difficult to attain equal properties in the longitudinal and transverse direction. It is especially difficult to obtain equal properties when it comes to the production of relatively thin films, e.g. films with a thickness of less than 20 xcexcm.
In this connection, DE 195 15 137 describes that the flat films described in EP-A 0 574 870 (respectively in DE 42 19 658 which corresponds to EP-A 0 574 870) have performance values in need of improvement particularly with regard to the mechanical properties transverse to the extrusion direction since drafting is performed exclusively in the longitudinal direction and as a result of the crystallinity connected with this. As a solution DE 195 15 137 describes a process in which first of all a cellulose tube is produced using the well-known blowing process, said tube being then cut open to a flat film.
Another possibility to improve the tenacity of flat films in the transverse direction is to stretch the film-like moulded solution in the air gap transversally to the direction of extrusion. A process of this kind is described e.g. in PCT-WO 97/24215 but it involves a great amount of efforts.
The present invention sets itself the task of providing a process for the production of cellulosic flat films according to the amine oxide process with which thin flat films with equal tenacity properties in the longitudinal and transverse direction can be produced without the additional need for stretching transversally to the direction of extrusion.
The present invention furthermore sets itself the task of providing a cellulosic flat film having been only monoaxially stretched which has an equal relationship between the tenacity properties in the longitudinal and transverse direction even with a larger film width and with lower thicknesses.
The task of the invention is resolved by the process in accordance with the invention by means of using an extrusion die which has an extrusion gap with a width b of 220 xcexcmxe2x89xa6bxe2x89xa6280 xcexcm.
Surprisingly it has been seen that by choosing the width of the extrusion gap correctly, given the same tenacity in the longitudinal direction, the transverse tenacity of the flat film can be improved quite considerably.
In this respect it is important to not keep the width of the extrusion gap too low since with low widths of the extrusion gap as is described in EP-A 0 574 870 poor properties in the transverse direction in the flat film result.
Thus for example the transverse tenacities of the flat films, which are extruded via a die with an extrusion gap with a width of b=150 xcexcm, are at a very low level. The transverse tenacities of flat films which are extruded in otherwise comparable conditions via a die with an extrusion gap with a width of b=220 xcexcm, are by comparison at a much higher level.
If, however, one advances to higher values for the width of the extrusion gap, e.g. to 300 xcexcm or more, this effect is not as pronounced. 220 xcexcmxe2x89xa6bxe2x89xa6280 xcexcm, and preferably 240 xcexcxe2x89xa6bxe2x89xa6260 xcexcm would appear to be the optimum range for the width of the extrusion gap.
The extrusion gap preferably has a length l of 40 cmxe2x89xa6l.
The film-like moulded solution can be drawn off in the air gap in the extrusion direction at a speed of 0.2 to 20-times the speed with which the film-like moulded solution emerges from the extrusion gap.
The length in the extrusion direction of the last parallel section of the die, before the solution emerges from the die, preferably equals more than 1 mm, and 5 mm to 20 mm are even better.
The task of the invention is further resolved by a monoaxially stretched cellulosic flat film obtainable by the amine oxide process, said film having a thickness d of less than 20 xcexcm, a width B of more than 30 cm, and a factor f of 65 or less whereby f is defined as f=d*(MD/TD), d is used in xcexcm and whereby MD stands for the tenacity of the film in the longitudinal direction (N/mm2) and TD for the tenacity of the film in the transverse direction (N/mm2).
The factor f preferably equals 60 or less in the flat film in accordance with the invention.
The flat films in accordance with the invention, therefore, reveal a very well-balance relationship between the tenacities in the longitudinal and transverse direction, even with very low thicknesses, although they are not stretched in the transverse direction. In combination with the greater width of the flat film in accordance with the invention this results in an advantageous flat film for numerous different applications.
The process in accordance with the invention is explained further by means of examples. In all the examples the flat films produced had a width of more than 30 cm.